Again, I would say if you want a top career in something like F1 designing parts of specialised prototype cars, then you'd be better of specialising in something Mechanical or Aeronautical, rather than doing a 'jack-of-all-trades' Motorsport degree, ideally up to MEng/MSc standard, and spending a few years in lower formulae getting experience/chartership.
Of course, there will be those that get through on a Motorsport degree (rare), those that get there without any qualifications at all (very very rare), and those that are 150% qualified for the job but don't get it because of other factors (interview ability, team work, coping under pressure etc). There is no right answer.
But I would say that if you want to design racing cars then a decent standard in a decent (non Motorsport) degree is the way forward. If you want to work at F1 level then a BEng is probably the best way to go, and you might get away with Motorsport if you don't have any illusions of getting past the Wishbone Fitter stage. The further down the ladder you aspire to (and bear in mind there aren't many jobs in F1, but an awful lot of applicants, so it would be VERY wise to lower your goals to FFord levels) the lower the qualifications of the best jobs in each branch.
Liking cars doesn't mean jack. The fact that you pissed around in school means you're unlikely to want to be away from home and friends for 340 days of the year doing 60 hour shifts in cold, damp paddocks. Motorsport, except at the very peak (where there are maybe 1000 jobs in the world), is NOT a glamorous or well paid job.
E: Design helps (at GCSE and A-Level). Further maths would help massively if it doesn't detract from everything else. I did Further Maths, but I wasn't clever enough to do it and 4 other A-Levels (this was before AS Levels meant giving up after one year), and ultimately I did worse at everything else. Had I been clever/committed enough, then further maths would have helped at degree level a lot - remember, a degree is 99% theory, so basic theoretical maths is perfect.